Cooperative Institute for Research in Environmental Sciences

Abstracts: 28

Fate and Transport of Organic Nitrogen in Aquatic Ecosystems

Sujay S. Kaushal and William M. Lewis, Jr.

Very little is known regarding the fate of organic nitrogen exported from terrestrial environments. Organic nitrogen may be an ecologically significant component of the nitrogen cycle in aquatic ecosystems, particularly those with low availability of inorganic nitrogen. We investigated the chemical and biological lability of dissolved organic nitrogen (DON) transported in two small streams draining watersheds, which receive low rates of atmospheric nitrogen pollution (ca. 3 kg/ha/y). As inorganic nitrogen declined in the streams, concentrations of DON increased linearly and became the most abundant form of nitrogen. Chromatographic separation of organic matter revealed that DON was comprised mostly of biologically reactive non-humic compounds whereas dissolved organic carbon (DOC) was primarily derived from more refractory humic substances. Organic carbon and nitrogen differed in their seasonal patterns of chemical fractionation and metabolism suggesting a decoupling of carbon and nitrogen cycling in these low nitrogen streams. Laboratory experiments indicated that the metabolism of DON remained high across a range of substrate quality and was influenced by the availability of inorganic N and P. Consumption of DON in incubations decreased exponentially as the availability of inorganic N increased in stream water over a seasonal cycle. Throughout the growing season, biologically available DON constituted a substantial proportion (up to 50-60%) of the total dissolved nitrogen exported from the watersheds. Our work suggests that metabolism of organic nitrogen may provide an important source of nitrogen to aquatic ecosystems which have been minimally affected by human perturbation.